It took more than a half century for the United Nations headquarters – with its sleek Secretariat tower and its General Assembly recognizable across the globe – to get its first major overhaul. And now with work under way, the goal is to keep the campus in operation until the $1.9-billion renovation is done in 2013.

So far, the multi-pronged campaign – which will renovate about 2 million sq ft of office and public spaces in five buildings, while also renovating, fitting out or building another 1.1 million sq ft of swing space and parking garages – is keeping to a revised schedule approved by the UN’s 192-member nations in 2007. It broke ground last year.

“The train has left the station,” says Michael Adlerstein, assistant secretary-general and executive director of the capital master plan.

And for now, after a round of value engineering, the team is slightly over the $1.88-billion budget the UN membership approved in 2007 and is aiming to get back in line, Adlerstein adds. “We were $219 million over budget in 2007 when we started working on this accelerated strategy,” he says, referring to the current plan that shaved two years off a prior timetable.

“And we have picked up about $120 million worth of savings, so we have another $100 million to go. And we’re getting good bids. In this unfortunate economic climate, funded projects do well, and we will be able to hopefully bring the project home within our budget.” About 35% of the project’s cost had been contracted as of late winter.

The project team will be working all across the 17-acre campus on tasks such as upgrading building systems and utilities, reconstructing office and public spaces, removing hazardous materials, improving structural systems and preparing the complex to meet sustainable design and code compliance standards.

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The complex was originally intended to support the diplomatic operations of 75 nations and host about 700 meetings a year, but it now has more than double the number of countries and hosts more than 8,000 events.

The current phase entails construction of a 175,000-sq-ft temporary building on the North Lawn that will serve two tours of duty as the swing space for the Conference Building and later for the General Assembly Building as those structures undergo renovations. That temporary building will be ready for its first move-in this fall, and its disassembly in 2013 will cap the entire project.

Crews are also finishing fit-outs of other offsite swing space, and the project team plans to move thousands of workers off campus this year in order to start on the 39-story Secretariat, which will get a gut renovation, including replacement of its nearly 60-year-old, single-glazed glass facade with high-performance, double-glazed curtain wall.

Nearly every major feature of the 840,000-sq-ft structure will get a fix during the project’s lifespan, but the work will begin with the relocation of a sensitive data center that was reprioritized late in the planning and separated out for special attention, Adlerstein says.

“We have to empty the building and then take the data center, which is in the middle of the Secretariat building, and move it to the basement,” he adds. “It has to be moved separate from the people because it’s fragile. It runs the UN systems all over the world.”

Adlerstein says that effort to move the data infrastructure will run from November to February.

Another current task is ensuring the work doesn’t disrupt the annual General Assembly debate in September, a major event on the organization’s calendar. Adlerstein says the team has reworked the staff-moving schedules to accommodate the event.

A guiding theme of the overall plan is making the complex more environmentally sustainable, which is a tall task in an aging facility. The team is aiming to meet or exceed the equivalent of the silver-level LEED standard. The new curtain wall is a big element, which complements other energy-saving building envelope features such as new roofing insulation and an automated interior shade-and-blinds system that will control heat gain and maximize use of natural light. The overall goal is to top 40% in energy consumption savings.

The budget dedicates more than $28 million to direct sustainable-design measures. Other examples are modern lighting, new indoor air-filtration equipment and more efficient building systems that would allow sustainable practices such as generators running on biodiesel fuels.

While the construction effort is now focused on the swing space fit-outs and temporary building completion, the work on the tower and other main buildings is slated to get into full swing next year and remain active throughout 2011 and 2012. Skanska Building USA of Parsippany, N.J., is the construction manager and is overseeing more than $1 billion of the budget for the construction effort.